Just my guess but i think there is no relation between MTBF and TBW. I think MTBF refer to any kind of failure to work properly and that could happen regardless if you are reading, writing or idle. That in turn means tere is no exact relationship between MTBF and the amount of data writen. MTBF seems to be a measure of reliability relevant in the enterprise business where you have many drives. For example 2m hours MTBF would mean something like if you had 230 drives in a server, you can expect one to fail each year.
Of course, it’s my theory and i could be wrong as well. Anyway i hope it can give you a clue. On the other hand, it would be great if someone at sandisk could give us further detail regarding those specifications…